apalache VS Formality

Anyone know of some good free software TLA+ model checkers? The "Other Tooling" mentions one alternative checker, https://apalache.informal.systems/, but that's all I could find. Thanks.

> How are those types any different than outright stating a behavioral invariant?

Because the behavior of programs can't be verified without executing the program, but types can be checked purely based on syntax. There is way less source code than runtime states of any non-trivial program.

I've asked this same question many times, the TLA+ way is much more expressive and _simpler_. But model checking is a way harder problem than type checking, in general. SMT solvers make this line blurry - in fact, have you heard of the SMT-based model checker for TLA+, [Apalache](https://apalache.informal.systems/)?. I haven't tried it out, but that should be way faster than TLC which just brute forces the state-space exploration.

I'm totally with you about TLA+ style spec properties, but it's a big theoretical hurdle to cross before they could be as efficient as types.

Also, my current work is using Kind as a foundation, the purpose of this language is exactly what you have asked for, give a check on https://github.com/uwu-tech/Kind.

Kind has a "how I learned to stop worrying and love the `Type:Type`" vibe. That doesn't make it invalid as a proof language. It just inverts the priority: instead of consistency being the default and expressivity being opt-in (as in Agda, with the `type-in-type` pragma), it is expressive by default, and consistency is an opt-in. I strongly believe that is the right way. We plan to add opt-in termination (thus consistency) checkers, it is just not an immediate priority, but the language is completely ready for that. About `Type in Type` specifically, keep in mind that there are consistent, interesting type theories that feature `Type in Type`. So it isn't problematic in itself, and removing it seems wrong.

About erasure, you can flag an argument as computationally irrelevant by writing `` instead of `(x: A)`. So, for example, in the [Vector/concat.kind](https://github.com/uwu-tech/Kind/blob/master/base/Vector/con...) file, `A`, `n` and `m` are erased. As such, the length of the vector doesn't affect the runtime. As a good practice, you may also write `f` instead of `f(x)` syntax for erased arguments, but that is optional.

> TL;DR -- I think the language looks nice, and the compile to JS (from what I read of the Formcore source) looks to be well done. Also, the docs that are present are well presented in a non-academic way that I find pretty readable.

Thanks for the kind words. We put a lot of effort on the compilers and, while there is still a lot to improve, I'm confident they're ahead of all the other languages, by far.

Kind is a functional, general-purpose programming language featuring theorems and proofs. It has the smallest core, a pretty solid JavaScript and Scheme compiler (seriously, check how clean is the generated kind.js), and a syntax that is a middle ground between Haskell and TypeScript, in an attempt to make it more accessible.

I'm writing CONTRIBUTE.md right now.